Charged particle detectors are well known, and typically comprise an electrode, means for establishing an electric field in the vicinity of the electrode so as to attract charged particles towards the electrode, and a transresistance amplifier for converting an input current made up of charged particles incident on the electrode into an output voltage.
The transresistance amplifier typically comprises an input resistance through which the input current flows so as to develop an input voltage, and a voltage amplifier that receives the input voltage and generates the output voltage.
The input voltage received by the voltage amplifier inevitably comprises both the signal component developed across the input resistance and a small noise component. The magnitude of the noise component is approximately constant, regardless of the magnitude of the signal component. From the point of view of the signal-to-noise ratio of the output voltage, therefore, it is desirable for the input resistance, and hence the input voltage, to be as large as possible.
The voltage amplifier has some input capacitance, which, together with the input resistance, forms a low-pass filter, the cut-off frequency of which is inversely proportional to the input resistance. From the point of view of the bandwidth of the output voltage, therefore, it is desirable for the input resistance of the amplifier to be as small as possible.
Given these conflicting requirements, previous transresistance amplifiers have been provided with a voltage amplifier with variable gain and several switchable input resistances, any one of which may be connected, typically by means of a relay, between the electron detector and the voltage amplifier. The variable gain of the voltage amplifier is necessary to accommodate the step changes in sensitivity resulting from the use of switchable input resistances.